Method and device for measuring an ion flow in a plasma

ABSTRACT

The present invention relates to a method for measuring an ion flow from a plasma to a surface in contact therewith, consisting of measuring the rate of discharge of a measuring capacitor connected between a radiofrequency voltage source and a plate-shaped probe in contact with the plasma.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and device for measuring aflow of positive ions from an ionized gas, or plasma, to a solid surfacein contact therewith, for example, a wall of a plasma reactor or asample to be processed. The present invention more specifically appliesto measuring an ion flow in an enclosure constituting a plasma reactorfor coating a sample with a thin layer, or modifying the structure orthe chemical composition of a surface by ion bombardment.

2. Discussion of the Related Art

FIG. 1 schematically shows, in a cross-sectional view, an example of aplasma reactor to which the present invention applies. It can be, forexample, a so-called capacitive coupling radiofrequency excitationreactor.

Such a reactor is made of a vacuum enclosure 1. Close to a first wall 2of this enclosure 1 is placed, on a wafer support 3, a sample 4 to beprocessed. Sample 4 is generally shaped as a disk having a surface 8directed towards the inside of enclosure 1 which constitutes the surfaceto be processed. Enclosure 1 is filled with a low pressure gas, forexample, of around a few tens to a few hundreds of millitorrs (a fewtens of pascals). Several means can be used to generate the plasma. Forexample, in a so-called "capacitive coupling reactive ionic etching"configuration, a radiofrequency voltage is applied to the wafer support.As shown in FIG. 1, the plasma can also be generated by means of asource 6 independent from wafer support 3. This source 6 is, forexample, for a d.c. voltage discharge, an electrode independent from thewafer support and supplied by a radiofrequency generator, an inductivecoupling radiofrequency source (often associated with the application ofa magnetic field) or a microwave source (which may be associated withthe application of a magnetic field). In the case of the use of a source6 independent from wafer support 3, the latter can be biased by aradiofrequency source 5 (FIG. 1) to establish a self-biasing and thusincrease the ion impact power on the surface to be processed.

In a plasma etching or deposition method, it is important to know thecharacteristics of the interaction between the surface to be processedand the plasma to be able to control the implementation of the method,especially, to control the deposition or etching rate, according to thedesired thickness of the deposition or depth of the etching. The flow ofcharged particles (ions and electrons) which arrive and leave thesurface to be processed enables to determine these characteristics whichdepend, notably, on the plasma used.

A so-called induced fluorescence method enables to determine, in certainlimited cases, the ion speed distribution function. However, such amethod does not allow to definitely determine the ion flow. Moreover,its implementation is particularly complex and very costly.

The present invention applies to a direct electrical measurement of theion flow in a plasma reactor.

Several methods are conventionally used to determine the characteristicsof plasma reactors based on electrical measurements.

A first, so-called "Langmuir probe", method consists in inserting, inthe middle of the plasma and thus away from the enclosure walls, a smallgenerally cylinder-shaped electrode. This electrode is connected,outside the enclosure, by a wire surrounded with an insulating sheath. Avariable voltage V is applied between the probe and the walls of thereactor and the current I in the wire is measured. The shape of thecurrent-voltage characteristic I(V) thus obtained enables to estimateparameters characteristic of the plasma, such as the ion and electrondensity, the electron temperature or the plasma potential. With amodeling, these parameters enable to obtain an estimate of the ion flowtowards the walls.

A so-called "planar Langmuir probe" alternative of this method consistsin placing, next to a wall (for example, wall 9 in FIG. 1) of enclosure1, an electrode shaped as a disk having a relatively large surface S(for example, a few square cm) with its rear surface directed to thewall coated with an insulating material.

FIG. 2 shows the shape of the current-voltage characteristic of such anelectrode in a plasma reactor. When a strongly negative voltage V isapplied, a saturation current Isat is reached. This current Isat is animage of the flow of positive ions Γ_(ion) since all the electrons arerepelled. The relationship which links current Isat to ion flow Γ_(ion),assuming that all the ions are ionized only once, is given by relationIsat=e.S.Γ_(ion), where e stands for the charge of an electron.

A disadvantage of Langmuir probe methods, which consist in measuring ad.c. current between the probe and the plasma, is that they no longeroperate when the probe is contaminated, in particular if the plasmadeposits an insulating layer on the electrode. This generally occurswith chemically complex gases (CF₄, SiH₄, CH₄, etc.) which quicklydeposit thin insulating layers on any surface in contact with theplasma.

A second method consists in sampling the ion (and electron) flow bymeans of a small aperture (generally having a diameter of approximately100 μm) in an electrode placed in the vicinity of an enclosure wall. Anelectrostatic filter placed behind the aperture enables one to separatepositive ions from electrons and thus to measure the transmitted ioncurrent. A disadvantage of such a method is that it requires acalibration of the transmission rate of the aperture and of theelectrostatic filter. Yet, depositions of thin layers on the filterresult in an alteration of the rate. The measurements are thus disturbedby these plasma-induced depositions, which makes them quicklyunexploitable and results in complete failure of the measurement device.

A consequence of the disadvantages of the methods described hereabove isthat conventional plasma reactors are generally characterized byoperating with a rare gas, for example argon, previously to anydeposition or etching method. The characteristics of a reactor in thepresence of a complex gas thus cannot be known otherwise than bymodeling.

Another disadvantage common to all known methods is that they do notallow any direct measurement of the ion flow during the processing of asample. They thus do not allow any control of a deposition or etchingmethod.

SUMMARY OF THE INVENTION

The present invention aims at overcoming these disadvantages byproviding a method for measuring an ion flow which can be implementedwhatever plasma is used. Specifically, the present invention aims atenabling the measurement of the ion flow in plasmas which deposit thininsulating layers.

The present invention also aims at providing a method which does notdisturb the deposition or etching method itself. In particular, thepresent invention aims at authorizing a control of a plasma depositionor etching method.

The present invention also aims at providing a device for implementingsuch a method which is of particularly simple implementation.

The present invention also aims at providing a device which does notrequire a calibration prior to the measurements. In particular, thepresent invention aims at enabling an absolute measurement of the ionflow.

The present invention further aims at providing a device which enables ameasurement of the homogeneity of the ion flow in the vicinity of thewall of the enclosure for receiving a sample to be processed.

To achieve these objects, the present invention provides a method formeasuring an ion flow from a plasma to a surface in contact therewith,consisting of measuring the discharge rate of a measuring capacitorconnected between a radiofrequency voltage source and a plate-shapedprobe in contact with the plasma.

According to an embodiment of the present invention, the measurementmethod consists of periodically supplying the probe with radiofrequencyoscillation trains and performing the measurement, between twooscillation trains, after the damping of the radiofrequency signal andbefore the potential of the probe is stabilized.

According to an embodiment of the present invention, the measurementmethod consists of performing a measurement of the potential variationacross the measuring capacitor.

According to an embodiment of the present invention, the method consistsof measuring the discharge current of the measuring capacitor by meansof a transformer interposed between the capacitor and the probe.

According to an embodiment of the present invention, the value of themeasuring capacitance is lower than the value of the capacitance of athin layer which may be expected on the probe.

The present invention also relates to a device for measuring an ion flowin a vacuum enclosure constituting a plasma reactor, including:

a probe internal to the enclosure and including a planar sensitivesurface;

means external to the enclosure for periodically supplying the probewith a radiofrequency voltage;

a measuring capacitor external to the enclosure mounted in seriesbetween the supplying means and the probe; and

means external to the enclosure for periodically measuring the dischargecurrent of the measuring capacitor or the potential variations acrossthe capacitor during its discharge.

According to an embodiment of the present invention, the probe iscomprised of a disk connected, by a substantially axial conductor, to aterminal of the measuring capacitor, the rear and lateral surfaces ofthe disk being surrounded with an insulator and a conductive sheathacting as a screen and a guard ring.

According to an embodiment of the present invention, the sheath isconnected to the supply source via a capacitor.

According to an embodiment of the present invention, the supplying meansare comprised of a radiofrequency voltage source which providesradiofrequency oscillation trains, the measurement being performedbetween two oscillation trains.

According to an embodiment of the present invention, the period of theradiofrequency oscillations is short with respect to the applicationtime of these oscillations, the application time of the oscillationtrains being long enough to establish a self-biasing voltage for theprobe and the time interval between two oscillation trains being longenough to enable the measurement.

These objects, characteristics and advantages as well as others, of thepresent invention, will be discussed in detail in the followingnon-limiting description of specific embodiments in relation with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2, previously described, are meant to show the state of theart and the problem to solve;

FIG. 3 schematically shows an embodiment of a device for measuring anion flow according to the present invention;

FIG. 4 is a cross-sectional view of an embodiment of a probe of ameasurement device according to the present invention such as shown inFIG. 3; and

FIGS. 5 and 6 illustrate timing diagrams of the measurement method of anion flow according the present invention.

DETAILED DESCRIPTION

For clarity, only the components of the device according to the presentinvention and of the plasma reactor which are necessary for theunderstanding of the present invention have been shown. Similarly, thetiming diagrams of FIGS. 5 and 6 are not to scale and the samecomponents have been referred to with the same references in thedifferent drawings.

FIG. 3 schematically shows an embodiment of a device for measuring anion flow according to the present invention.

The device is comprised of a probe 10 meant to be placed near a surfacetowards which the ion flow is desired to be determined, for example,wall 9 (FIG. 1), of a vacuum enclosure (not shown) of a plasma reactor.A sensitive surface 11 of probe 10 is directed towards the inside of theenclosure. According to the present invention, probe 10 is connected toa measuring capacitor 12 having a capacitance C_(m). A first terminal Aof capacitor 12 is connected to a first terminal of a radiofrequencyvoltage source 13 having a low output impedance (typically 50 ohms), asecond terminal of which is connected to the reactor walls and, with thelatter, to a reference potential, generally the ground. In the casewhere the reactor walls are made of a non-conductive material, anelectrode with a larger surface than the probe is added to be used as areference for the measurements. A second terminal B of capacitor 12constitutes both an input terminal of the device to be connected toprobe 10 and an output terminal of the device towards measuring means,for example an oscilloscope 14.

FIG. 4 is a cross-sectional view of an example of an embodiment of probe10 equipping a measurement device according to the present inventionsuch as shown in FIG. 3.

Probe 10 is comprised of a planar disk 20 meant to be connected, by aconductor 21, to terminal B of the measurement device. Conductor 21 thusruns through the wall (for example, 9) near which probe 10 is placed.The surface of disk 20 directed to the inside of the enclosureconstitutes the sensitive surface 11 of the probe. The probe is,preferably, surrounded with a guard ring. This guard ring, intended toavoid the influence of edge effects, is for example comprised of aconcentric conductive disk 22, larger and thicker than disk 20. Disk 22is provided with a cavity in which disk 20 is inserted. The lateralsurfaces and the rear surface of disk 20 as well as conductor 21 areinsulated from disk 22 by an insulator 23. For disk 22 to better ensureits function of guard ring, it is preferably electrically connected, ina way not shown, to terminal A by a capacitor of value C_(g) (notshown). For the potential of the guard ring to be always close to theprobe potential, C_(g) will be chosen such that: C_(g) /C_(m) =S_(g)/S_(m), where S_(g) and S_(m) stand for, respectively, the surfaces ofthe guard ring and of the probe.

According to the present invention, probe 10 is periodically excited byradiofrequency voltage source 13. In other words, source 13 suppliesoscillation trains at regular intervals between which the discharge ofmeasuring capacitor 12 can be observed.

FIG. 5 illustrates the shape of the signal applied to terminal A ofcapacitor 12 by source 13. This drawing shows, in the form of timingdiagrams, potential V_(A) of terminal A. The periodicity T_(h) of theoscillation trains corresponds, for example, to a frequency f_(h)=1/T_(h) between 1 and 20 kHz. The radiofrequency oscillations last aperiod T₁ corresponding, for example, to approximately half (T_(h) /2)the period of the oscillation trains. The measurements are performedwithin the time interval T₂ between two oscillation trains of durationT₁. A signal such as shown in FIG. 5 is, for example, obtained by meansof a source 13, the output of which is chopped at a frequency f_(h).

Radiofrequency oscillation frequency f_(o) is, for example, includedbetween 1 and 20 MHz. For a radiofrequency excited plasma, the value offrequency f_(o) will be kept sufficiently distant from the frequency ofplasma excitation (by source 6 of FIG. 1) to avoid the occurrence ofinterferences with the plasma potential. As a particular example, for aplasma generated by means of a capacitive coupling generator having afrequency of approximately 13.5 MHz, one will choose, for frequencyf_(o), a frequency between 12 and 15 MHz.

Under the effect of the oscillations issued by source 13 and of thenon-linearity of the current (according to the voltage applied) suppliedto the probe by the plasma, the mean flow of electrons towards the probeexceeds, initially, the mean flow of positive ions, which causes theloading of capacitor 12. The mean value of the oscillations of thepotential of terminal B will decrease until it reaches a negative valuecorresponding to a potential V_(bias) where the electron flow isdecreased to reach a value identical to that of the ion flow, and thuswhere the mean resulting current in probe 10 is zero. This potentialV_(bias) results from the conventional self-biasing effect existing in aplasma.

FIG. 6 illustrates this operation and shows, in the form of a timingdiagram, the potential of terminal B of the capacitor 12 in the presenceof the plasma. Self-biasing potential V_(bias) corresponds,substantially, to half the peak-to-peak amplitude V_(CC) of theradiofrequency oscillations.

At the end of the oscillation train, that is, when the radiofrequencysignal is cut-off, terminal B of capacitor 12 is, after the oscillationshave damped, at self-biasing potential V_(bias). Probe 10 then beingbiased at a strongly negative potential, it is not able to captureelectrons. However, the ion flow arriving on surface 11 of probe 10remains unchanged and starts to discharge capacitor 12. Thus, thepotential of terminal B of capacitor 12 will increase linearly until itconverges to a floating potential which corresponds to a value V_(f)where the ion flow and the electron flow compensate each other.

According to the present invention, the discharge rate of capacitor 12is measured during the linear period where the current is only comprisedof the ion flow (the electron flow is zero). These measurements areperformed either by observing the time derivative dV_(B) /dt of thepotential at terminal B, or by observing the current I_(B) flowingtowards capacitor 12 by means of a transformer interposed between probe10 and capacitor 12. For this purpose, for example, an oscilloscope 14or a specific signal processing circuit is used. The measurements areperformed after the radiofrequency oscillations have damped and beforethe signal variation stops being linear, that is, before the potentialat terminal B comes close to floating potential V_(f).

During the discharge of capacitor 12, the variation of the potential ofterminal B follows, as a first approximation, the following relation:

    dV.sub.B /dt=e·S.sub.m ·(Γ.sub.ion -Γ.sub.e)/C.sub.m,

where Γ_(ion) and Γ_(e) stand for, respectively, the ion flow and theelectron flow and where e stands for the charge of an electron.

Electron flow Γ_(e) varies according to potential V_(B) and becomes zerowhen potential V_(B) is strongly negative, as in the case of a planarLangmuir probe.

The amplitude of the radiofrequency signal is chosen to be high enough(for example, of around a few tens of volts) for self-biasing potentialV_(bias) to be negative enough to prevent the electrons from beingcaptured by probe 10 during a time sufficient to perform themeasurement. This amounts to saying that amplitude V_(CC) of theradiofrequency signal is chosen to be clearly higher than the electronictemperature as expressed in electron-volts.

Thus, as long as the electrons are repelled by the probe since it is ata potential negative enough with respect to potential V_(f), electronflow Γ_(e) on the probe is zero and the discharge slope of capacitor 12is proportional to ion flow Γ_(ion).

By measuring this slope, by means of oscilloscope 14, the ion flow canbe inferred from the following relation:

    I.sub.B =C.sub.m ·dV.sub.B /dt=e·S.sub.m ·Γ.sub.ion.

The presence of a plasma depositing a thin insulating layer does notaffect the operation of a device according to the present invention.This thin layer appears, from the electrical point of view, as acapacitance C_(i) (not shown) in series with capacitor 12 betweenterminal B and the plasma. The consequence of the presence of thiscapacitance is that the potential measured on terminal B does notcorrespond to self-biasing potential V_(bias), but to a fraction of thispotential due to the series association of capacitor 12 with capacitanceC_(i) of the insulating layer.

The relation which links the potential of terminal B to potential V_(S)of the probe surface in contact with the plasma, at the time when theradiofrequency signal is cut-off, is:

    V.sub.B =V.sub.S ·C.sub.i /(C.sub.m +C.sub.i).

The value of potential V_(S) at the time when the radiofrequency signalis cut-off is still V_(bias). The initial absolute value of potentialV_(B) will thus be reduced. However, as long as potential V_(S) remainssufficiently negative, the ion flow and thus the current (identical)flowing through capacitances C_(m) and C_(i) remains unchanged. Theeffects of capacitance C_(i) of the thin insulating layer are to reducethe absolute value of the potential V_(B) obtained at the end of theradiofrequency oscillations, to reduce the charge accumulated bycapacitor 12 having a capacitance C_(m) and to reduce the discharge timeof capacitor 12 and, accordingly, the linear period during which themeasurements can be performed. Conversely, the initial value of the timederivative dV_(B) /dt remains unchanged and the discharge rate alwaysis, at the beginning, proportional to ion flow Γ_(ion) for a high enoughamplitude V_(CC) of the radiofrequency signal.

Preferably, capacitance C_(m) of capacitor 12 is chosen to be lower thanthe expected capacitance of the thin insulating layer likely to bedeposited on probe 10 by the plasma. This has the advantage ofincreasing the time for which the potential variation on terminal B isexploitable.

The choice of the value of capacitor 12 depends on the electric noiselevel of the installation with which the device is associated and thedesired exploitable discharge time. Indeed, the greater capacitanceC_(m), the slower the discharge of measuring capacitor 12.

If the value of capacitor 12 is too high, the measured potentialvariation dV_(B) /dt is too slow and the measurements risk being alteredby noise.

If the value of capacitor 12 is too low, its discharge risks being toofast and not leaving enough time to reach a balance of the space chargeregion in front of the probe, thus resulting in an error in themeasurement. The time required for this balance is given by theion-plasma period: t_(ion) =(M_(i).ε₀ /n.e²)^(1/2), where M_(i) standsfor the mass of the ion, ε₀ stands for the space permittivity and nstands for the ion density in the plasma.

The value of capacitor 12 is, for example, chosen to be around a fewnanofarads. Such a value meets the condition relating to the capacitanceof a possible thin insulating layer deposited by the plasma. Indeed, thecapacitance of a thin insulating layer with a relative dielectricconstant ε_(r) of 4 and a thickness of 0.1 μm is around 177 nanofaradsfor a probe of 5 cm².

The period T_(o) =1/f_(o) of the oscillations will be kept short withrespect to the time T₁ of application of the oscillations, time T₁ willbe kept long enough to enable a self-biasing of the probe at potentialV_(bias) and time interval T₂ between two trains of oscillations will bekept long enough to enable the measurement of the discharge rate ofcapacitor 12.

An advantage of the present invention is that it applies to any plasmachemical composition. Only a very strong chemical attack (etching) ofthe material(s) forming the probe or a deposition of an insulating layerhaving a thickness such that it no longer enables to reach aself-biasing of the probe limits the operation of the measurementdevice.

Another advantage of the present invention is that the results obtainedare independent from the chemical nature of the ions gathered by theprobe. Indeed, only the electric current is measured, and thus the totalflow of positive ions.

Another advantage of the present invention is that it enables control ofa deposition or etching method during its implementation. A probeaccording to the present invention can be placed in the vicinity of anenclosure wall other than that next to which the sample to be processedis placed. The result of the measurement performed by the deviceaccording to the present invention can then be used to control themethod. If the device detects a slight variation of the ion flow(acceleration or slowing down of the discharge rate of the measuringcapacitor), it can issue an order enabling modification of theradiofrequency or microwave gas excitation signal in order to modify theplasma. If the device detects an abrupt fall of the ion flow, it cangenerate an alarm indicating that the enclosure walls are contaminated.It should be noted that several probes associated with severalmeasurement devices can be distributed next to the enclosure walls tohave measurements in different regions of the enclosure.

Another advantage of the present invention is that it enables one tocheck, in an enclosure characterization phase, the homogeneity of theion flow in the region of the enclosure meant for receiving a sample tobe processed and this, with any plasma. For this purpose, several probesand measurement devices according to the present invention aredistributed next to the wall meant for receiving, in normal operation,the sample to be processed. The interpretation of the measurements givenby the different devices enables one to draw a map of the distributionof the ion flow next to the wall considered.

According to the present invention, the sensitive surface 11 of probe 10is relatively large (around several square centimeters). Indeed, thelarger the probe surface, the larger the current gathered for a givenion flow. This enables the improvement of signal-to-noise ratio and thetime resolution of the probe. Moreover, the edge effects and the effectsof possible insulating layers are minimized with a large probe. In anapplication where several probes are used to establish a mapping of theion flow with respect to the position, the size of the probes will beadapted (by limiting it) to obtain an adequate spatial resolution.

Another advantage of the present invention is that the measurements arenot affected by magnetic fields as long as the gyromagnetic radius ofthe ion is lower than the size of the probe and as the amplitude andorientation of the magnetic field does not prevent the electrons fromreaching the probe, and thus from charging capacitor 12 to establish aself-biasing. The size of probe 10 can thus be adapted to the maximumexpected magnetic field. For example, for a probe whose disk has adiameter of 1 cm, the measurements will not be disturbed by magneticfields lower than 1000 Gauss.

According to a variant of the invention, wafer support 3 (FIG. 1) can beused as an ion flow probe. In this case, the radiofrequency supply(generator 5, FIG. 1) which is used to establish the self-biasing of thesubstrate and which, in the case of a capacitive coupling reactive ionicetching, feeds the plasma, is chopped and then plays the role of aradiofrequency voltage source 13 according to the present invention.Means similar to those described in relation with FIG. 3 are used tomeasure the ion flow. Although the implementation of such a variantrisks, should it be used, disturbing the processing of the substrate, itenables one to study the possible difference between the flows on thewalls and on the wafer support, which can be due to a non-homogeneousdistribution of the ion generation regions.

It should also be noted that the present invention enables one todetermine the potential of the plasma and the electronic temperature.Indeed, by analyzing the current-voltage characteristic of terminal Bwhen potential V_(B) comes close to floating potential V_(f), the probethen can provide parameters characteristic of the plasma, as aconventional Langmuir probe would do.

Of course, the present invention is likely to have various alterations,modifications, and improvements which will readily occur to thoseskilled in the art. In particular, the materials, dimensions,capacitances and frequencies indicated as an example can be modified,especially according to the plasma reactor for which the device ismeant. Further, although reference has been made in the foregoingdescription to a reactor generating the plasma by means of a capacitivecoupling radiofrequency generator, the present invention applieswhatever the way of exciting the gas, be it d.c., radiofrequency ormicrowave. Besides, the present invention also applies to a measurementof flows of charged species present in plasmas other than positive ions,such as aggregates of nanometric sizes or positively-charged dustparticles.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andthe scope of the present invention. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting. The present invention is limited only as defined in thefollowing claims and the equivalents thereto.

What is claimed is:
 1. A method for measuring ion flow from a plasma toa surface such as a probe, comprising the steps of:applying a radiofrequency voltage to a first terminal of a measuring capacitor connectedbetween a radio frequency voltage source and a plate-shaped probe incontact with said plasma, whereby said capacitor becomes charged at aself biasing potential of the plasma; periodically supplying the probewith radio frequency oscillation trains by means of said radio frequencyvoltage source and performing the measurement of said discharge rate,between two oscillation trains, after the damping of the radio frequencysignal and before the potential of the probe is stabilized; andmeasuring a discharge rate of the capacitor when discharged by the ionflow arriving on the probe.
 2. A measurement method according to claim1, wherein said measurement of said discharge rate is performed by ameasurement of the potential variation across the measuring capacitor.3. A measurement method according to claim 1, wherein said measurementof said discharge rate is performed by a measurement of the dischargecurrent of the measuring capacitor.
 4. A measurement method according toclaim 1, wherein the value of the measuring capacitance is determined byan external measuring capacitance and the thickness and dielectricconstant of a thin film which may be present on the probe.
 5. Ameasurement method according to claim 1, wherein the measuring is usedto detect the deposition of insulating layers on the reactor walls.
 6. Ameasurement method according to claim 1, wherein the measuring is usedto determine the electronic temperature of the plasma.
 7. A device formeasuring an ion flow from a plasma to a surface such as a probe in avacuum enclosure constituting a plasma reactor, including:a probeinternal to the enclosure and including a planar sensitive surface, saidprobe comprising a disk connected, by a substantially axial conductor,to a terminal of the measuring capacitor, the rear and lateral surfacesof the disk being surrounded by an insulator and a conductive sheathacting as-a screen and a guard ring; means external to the enclosure forperiodically supplying the probe with a radio frequency voltage; ameasuring capacitor external to the enclosure mounted in series betweenthe supplying means and the probe; and means external to the enclosurefor periodically measuring the potential variations across the measuringcapacitor during its discharge.
 8. A measurement device according toclaim 7, wherein the sheath is connected to the supplying means via acapacitor.
 9. A measurement device according to claim 7, wherein thesupplying means are comprised of a radio frequency voltage source whichprovides radio frequency oscillation trains, said measurement of thepotential variations being performed between two of said oscillationtrains.
 10. A measurement device according to claim 9, wherein theperiod of the radio frequency oscillations is short with respect to theapplication time of these oscillations, the application time of theoscillation trains being long enough to establish a self-biasing voltagefor the probe and the time interval between two oscillation trains beinglong enough to enable said measurement of the potential variations. 11.A method for measuring ion flow from a plasma to a surface such as aprobe, comprising the steps of:applying a radio frequency voltage to afirst terminal of a measuring capacitor connected between a radiofrequency voltage source and a plate-shaped probe in contact with saidplasma, whereby said capacitor becomes charged at a self biasingpotential of the plasma; periodically supplying the probe with radiofrequency oscillation trains by means of said radio frequency voltagesource and performing the measurement of said discharge rate, betweentwo oscillation trains, after the damping of the radio frequency signaland before the potential of the probe is stabilized; and measuring adischarge rate of the capacitor when discharged by the ion flow arrivingon the probe, wherein the value of the measuring capacitance is lowerthan the value of the capacitance of a thin layer deposited by theplasma on the probe.